This is to take a closer look at what is known as microphonics, the phenomenon that is named as such because this is the natural way that microphones work. They react to sound pressure changes and create a signal that is send that to the amplifier, so we get a system with what is known as positive feed back which is by design microphonic. The word is used for other systems involving an amplifier, this includes guitar pickups that react to vibration of other things than the actual strings, as perturbation caused by mechanical vibrations reaching the parts of a pickup and of such a nature that it will cause the pickup to create an electrical signal on its output wires, basically what is it and how is it generated. The subject for this section is this and what is the pickup reacting to and how is the output signal created because of this. I was hoping to find some scientific treatment of this subject, especially for guitar pickups, so I searched “microphonic pickups” and what I found was pretty disappointing because there was nothing of value, nothing but pseudo-technical BS and lots of myths. Nothing of any substance. So what does one do in that situation, he develops his own theory. The following is my take on the subject.
Introduction
I have previously discussed this and to some extent jumped over where the fence was the lowest dismissing the waxing of pickups, especially the coil so that the wires could not move as the source of the feed-back basically stating that the wires moving does not create a change in coil area or anything else that can create an output voltage. I believe that we need to look at the entire construction of the pickup because mainly a change in magnetic flux density, B, in the the medium inside the coil(s) can generate an output voltage. So here is our starting point, a mechanical vibration is transformed into undesired signals that are unwanted output of the amplifier.
Experiments
I started out with a humbucker and mounted it on a piece of 3/4″ thick maple plywood, mounted a piezo electric transducer on the wood surface, then I made a pencil size hammer out of a small piece of dowel with a 3 mm thick brass “shaft” also, I took a 5″ long 5 mm dowel as a link between the hammer and the pickup, the “stick”. I hooked the pickup output up to an oscilloscope to measure any output from the pickup along with the signal from the piezo transducer.
At first, you tap on the pickup to see what kind of signal you get out, tapping in different places, including on the plywood gives you different types of signal. Now, we are looking for the source of the signal, we know it must me something that causes a B change in order to give any output. So we get a signal by doing that and knowing it is magnetic, we might as well eliminate anything else by removing the bar magnet from the PU, doing this and tapping with the hammer, even with some force, yields nothing, no output from the PU, so we can put the magnet back in and continue. This PU does not have any wax potting around the magnet, I need to take it further apart to see if the coils are potted, so that is unknown at this point. It comes as no surprise that the source is connected to the magnet and the B flux density created by it.
In the following, we will take a look at a few graphs recorded as described above, the first set is for the pickup named PU 30, the one described earlier. We tap directly on the pickup itself as well as the plywood it is mounted on to se the reaction to both type perturbations.
Fig 1 shows the results of tapping directly on the pickup in the center basically impacting both bobbins. The test was repeated hitting one of the pole pieces directly, rendering the same result. Notice how the oscillation is shaped, large amplitude tapering off towards the end, what is known as a damped oscillation. The frequency of the first cycle is indicated on the top, 7812 Hz. This frequency is very close to the electrical circuit’s resonance frequency. Not much of a surprise since this is the output of the pickup, notice that the first peak is about 1.5 V, close to ten times the normal output of this pickup, which shows how much more powerful a gentle tap is compared to the oscillating string. The orange graph id the output of the piezo transducer, very small compared to the PU output, which means that not a lot of energy is transmitted to the support, so even if the pickup is bolted to the guitar body not much is transmitted.
Next step is to tap on the wood that the pickup is mounted to see what the piezo signal looks like, this can be seen in Fig 2.
So far the pickup under test was a normal construction humbucker with a ceramic bar magnet, except that both coils were equipped with pole pieces not the screw and pole piece type. Besides that, we can consider this pickup of a typical, traditional humbucker. The burning question is what is causing the output signal when the PU is tapped with a hammer directly on the pickup body. As we can see, now that we tap on the wood instead, we get a sizable output from the piezo transducer and almost nothing from the pickup, pretty much confirming the statement above. Any vibration of the wood (guitar body) is small even when the pickup is bolted to the wood. Notice the frequency indicated, this is the resonance frequency of the system when the perturbation is to the wood and now we get about half the frequency as above, indicating a system where the mass of the plywood is the dominant factor.So far the pickup under test was a normal construction humbucker with a ceramic bar magnet, except that both coils were equipped with pole pieces not the screw and pole piece type. Besides that, we can consider this pickup of a typical, traditional humbucker. The burning question is what is causing the output signal when the PU is tapped with a hammer directly on the pickup body.
Now pondering the results from above, my thought was to try something else, a pickup of a completely different construction, a Lace Alumitone pickup, also a humbucker. This is the pickup that has been tested before, known as PU 10. This pickup was already mounted to a piece of wood, but this time it is a much smaller piece of poplar, 1/4″ thick and not much bigger than the pickup. This time the PU is also bolted to the wood. This time there was no piezo electric transducer involved, but we have already seen the effects of that, besides this piece of wood is too far from mimicking a guitar body, so the omission is inconsequential.
The result of tapping on one of the two magnets is shown in Fig 3. Another tap was performed on the aluminum frame near the small transformer with the same result, a frequency around 4464 Hz, possibly the electrical resonance frequency of PU 10, I do not remember if it was measured, mainly due to the completely different design. To move on, the PU 10 output was also measured when tapping on the wood, in one corner. Fig 4 shows the resulting waveform.
Something different about this waveform, it is what is known as a “pulse packet”, not the normal damped waveform. Two things should be noticed, the frequency indicated 4672 Hz is about the same as the one shown in Fig 3 and the amplitude is much lower for the wood tap. Just as in the case of PU 30.
Both PUs were tested when plugged into a small combo amp and the result was that both would squeal when the gain was cranked all the way up and the PU was held as close as I could the the speaker. It was just the pick as mounted as described earlier and it was obvious that the magnetic interaction with the speaker was quite intense.
So what can we conclude so far. Regardless the difference in construction the reaction of the two PUs were very similar. Even though the obvious main difference is the fact that PU30 has many thousands of turns of thin copper wire and PU 10 only has one (1) turn per “coil”, which leads me to think tha it is NOT the copper wire moving and stated by many people. I have discussed the whole coil and copper wire issue elsewhere and concluded based on the analysis then that it could not be the copper wire moving, because it would not create a sufficient change in B to give any output of the PU.
Well, in lieu of the findings, what is it that is causing a pretty healthy output when tapped. Now, as a scientist, I keep thinking about these things (along with everything else) and that typically leads to something, an explanation or a cause of something that can the origin of the observed. So I came up with a thought, something from the back of my mind from my earlier career working on magnets and magnetism: Magnetostriction!
This is the effect of magnets or ferromagnetic material reacting to mechanical perturbation or impact so that tapping the rigid structure of a pickup that is very sensitive to changes in magnetic flux will react to even small changes in a magnet or ferromagnetic material due to mechanical disturbance such as a hammer tap! The interesting part is that the effect works both way, take a larger power transformer, you have most likely been able to hear how it hums when power is transferred. This power transfer is via magnetics using a laminated core as medium and the current in the electric wire coils will generate a varying magnetic field that will make the ferromagnetic material laminate in the transformer vibrate slightly through the effect of Magnetostriction. This current is changing with a frequency of 60 Hz, the sound you hear from the laminates!
As you have seen above, a piezo transducer was used to compare outputs from it and the pickup. The reason for considering magnetostriction as the answer here, there is a similar effect that is in play generating the voltage output from a piezo transducer. In this case it is a mechanical impulse the the material that makes the material generate an electric signal. Here it is generated directly out of the material, with the pickup it is different, the generation is indirect, as we shall see in the following.
If we stick with the myth a little bit, how does wax potting help if it is not the copper wire vibrating? Wax potting will also restrict the magnet in moving and therefore remove some of the sensitivity to mechanical stress (a hammer tap). Wax potting of a Lace like PU10 is both impractical and unnecessary, but PU30 could benefit from having its magnet dampened by wax. For good measure, here is a picture of the two pickups in this experiment, Fig 5.
A few comments concerning the picture in Fig 5. In case you are not familiar with this type pickup, the back metal (Al) form two loops of a single turn, the magnets for each of these turns is the grey rectangles. Each of these loops are interconnected in a way that they will act as the two coils in a humbucker pickup. Now, one turn per coil will not give you much output signal so there is a transformer that is coupled such that the signal from the coupled primary loops is transformed to a larger output signal in order to get a level similar to other humbuckers. Now here is something really clever, the two aluminum loops basically is the entire structure of the pickup and since this, one-piece, aluminum chassis is isolated electrically from the secondary of the transformer, the output signal, this structure can be connected directly to a shield that is electrically independent from the pickup signal (the way it always should be).
Feed-back and microphonics
A completely different thing is feed back caused by a vibrating string, this has been discussed elsewhere on the main Pickup Science page. An example I have seen in a video (yes, on YouTube) that was made to demonstrate microphonics. The guitar had all its strings on it and it was a semi-hollow body to boot! You can probably see where I am going with this, yes exactly, the microphonics was demonstrated by the guy shouting at the pickup on the guitar and got that same sound out of his amp. Yes you see the ridiculous in the situation, I mean the hollow body will resonate and the strings will vibrate because they are being shouted at, so it is a combination of strings and hollow body resonance that is creating the output signal from the pickup, not any microphonic effect of the pickup. Just to prove the point, I tried to shout loudly at the pickups in Fig 5, no reaction to see on the scope, I mean NONE. Microphonic my ass!
Magnetostriction, further information
In case you are more interested in Magnetostriction, try Wikipedia. I looked it up, there is a pretty good coverage of this and related phenomena.
So, where are we now?
At this point I believe that I have reached the point where I can say that the movement of coil or wire is not the cause of the the unwanted output from the pickup. If you have read the Pickup Physics page, I have discussed it there without getting into too heavy math, which I just might have to at some point, we shall see! As I said, I am convinced that it is magnetic and that the mechanical impact to the pickup is causing the magnetic flux to change momentarily and that the damped oscillations you observe on the oscilloscope (the PU output) is striking the resonance frequency of the pickup electrical circuit, the coil inductance and capacitance, or a derivative of this frequency, harmonic or sub-harmonic. The phenomenon is quite well known and the reaction from the pickup is pretty much the definition of resonance. Resonance frequency is therefore sometimes called natural frequency of a system. Now, I explained the mind of a scientist earlier, you are never satisfied that you arrived at the correct answer, so you just have to try one more experiment to further prove your point. That’s me!
So I found an old Fender pickup from the mid 1980es one the I have used earlier, PU 17, it has been part of other experiments. The nice thing about this is that you can remove the cover and have direct access to the copper wire coil. Furthermore, I looked to see if it had been wax potted, I did find some traces of wax, but it is uncertain if it was potted and I am not going to poke around in the copper coil winding to look for any wax, it is after all me oldest pickup, I think and it still works, obviously. In addition to that I made some “dampers” for the pickup, they consisted of 1/4″ rubber foam with one side adhesive and attached these to PU length square dowels 3/8″. With a C-clamp, I could attach these to the pickup from two sides to act as dampers for the PU coil, making sure it would be unable to move at all. As far as actual testing goes, I have tried a few things in the way of tapping in different points, but I have been able to boil it down to tapping on the magnets only with my little hammer and the round dowel of same diameter as the pickup magnet (or pole piece).
First experiment was with the PU 17 as is, cover removed and oscilloscope probe attached to the two wires. This represents, from the scope’s point a very large impedance, which means the pickup is left to its own devices, free to oscillate. This means, that what we see on the scope screen is purely pickup created, specifically amplitude, frequency and damping.
Second experiment was with the dampers attached so firmly that there is no way the coil wires could move and again the pickup response was tested by tapping on one of the magnets, poles 3 and 4 are preferred, they are the middle ones in the pickup. The resonance is still very pronounced and unchanged from the first experiment. Now, I have to stop here and say that resonance in a system is unwanted and something that you almost at all cost is trying to avoid without shrinking the useful frequency range. This was discussed on he Pickup Electronics page, how “Volume Pot” values change the frequency spectrum of a pickup. It does not have to be the pot, it could be a load resistor placed across the output of the pickup, something that was touched on in the Active Pickup section.
So the third experiment was to move the dampers and add a resistor across the output to see how much it would dampen the oscillations. Placing a 150k resistor there, basically got rid of the unwanted resonance.
Several experiments were conducted in order to get a consistent and repeatable impact on the pickup such that the results from the three experiments could be compared. Just tapping on the pickup pole with hammer and stick was far from giving uniform results, without that, they would be useless. So a 12″ brass tube was used with a short poplar plug attached to the end. The tube was 5 mm ID and 8 mm OD. The drop weight was a 4″ long 4 mm diameter Aluminum rod that could fall freely and apply the impact to the pickup pole piece on which the wood plug was centered. The result of this test can be seen in Fig 6.
In general, Fig 6 shows that the damping of the pickup coil wires did not gave much if any effect. What did have an effect was the 150k resistive load that was placed across the output of the pickup and the result of placing this resistor there is that we practically speaking get rid of the harmonic peak which was one of the objectives of this exercise. Placing a load there has a significant influence on the harmonic oscillations without affecting the frequency range.
It looks like it might be appropriate to show a picture of the “tools” used to make the explanation from above complete, they are shown here in Fig 7.
In the bottom in Fig 7 you see the “stick” the piece of poplar dowel used with the “hammer” right above it. The 4″ brass rod next to the hammer got onto the picture by mistake, I do not remember using it for anything! The 2 damping devices are shown above the hammer and stick. To the left we see the 12″ copper tube and the aluminum drop weight that was used to drop from the top of the tube and the impact when hitting the little piece of wood dowel at the end would transfer to the pole piece of the pickup, seen in Fig 8.
The pole piece that was the test object in this test was the one to the left of the center as it represents the most common pattern since it is surrounded by two other pole pieces such as there are no “end effects” as described elsewhere.
I picked a 150k resistor as an example in Fig 6, but I tried a few other values in the simulation type that I discussed on the Guitar Electronics main page. My aim was to find a standard resistor value that would give me a “flat” frequency characteristic, the result is shown in Fig 9.
This resistor is not much different that the 150k used in Fig 6 so the result would probably not differ much. This resistor value is the one I would use in connection with PU 17 if we had to convert it into an active pickup in order to dampen the resonance in order to avoid the ill effects of this frequency. It is not easy to see in Fig 10, but the “knee” of the curve is still past 6 kHz so there would not be any impact on the sound spectrum at all also be cause the active part does not change this frequency range because its input impedance is very high.
Please notice that the frequency scale (horizontal) is logarithmic so it is looks like the “knee” is at a higher frequency than that. It is possible, of course, to raise the knee frequency by using a higher resistance resistor, such as a 150k, that will give you a higher frequency and a little bit of high end boost.
The “difficult ” part
We have seen the equation for induced voltage in the coil elsewhere and how this voltage depends on the the change in magnetic flux and a simple version where the the flux depended on a constant flux density and a constant flat area, A. And in addition to that, the voltage induced is proportional to the number of turns in the pickup coil, N. To be more general, we have to look at the more accurate equation because, as we have seen in the section “Permanent Magnets”, the B field is not constant across the surface of a coil turn and the angle between the surface and the direction of the B field is not a right angle, it is not even constant within the area in question. In the general format, both B and A are treated as vectors which is common in physics and mathematics. The fact that it is a vector means that the entity is characterized by both value and direction, so it is like an arrow. For the B field it is simply the strength of the field in a certain spot and the direction of the the B. For the area A, in case it is not a flat, must be divided up into small areas of the size of dA and the area direction is perpendicular to this little area dA, so B now becomes B and dA becomes dA where the bold indicates that the entity is a vector. This means that the equation as presented elsewhere, the simple equation, is this
Φ = B . A
If we expand it to include what we discussed above, it will look like this
Φ = Σ B . dA
Now, the dot in the equation takes a new meaning since we are talking about vectors, it is called the “dot” product, this product is a scalar, meaning the sum indicated by Σ is a scalar, a plain number, not a vector. The conclusion of that is that the equation will now look like this
Φ =Σ B . dA . cos(α)
where α is the angle between B and dA, the sum indicates of course that every such product must be summed over the entire area of the coil loop. The B that determines the output voltage is the B surrounded by the pickup coil and if we look at the area of the coil A, you will see that the two vectors are parallel, α = 0, meaning that the factor cos(α) = 1. If you have read the Permanent Magnets page you will know this from most of the figures on that page. We have discussed before that the area of a winding in the coil will vary depending on its location in the coil. Inner windings will have a smaller area that outer windings in the coil, something we overcame by assigning an average area A for this purpose. This can be looked at in both ways, but the end result is the same. We are ultimately interested in V, the output voltage, and how any disturbance of any of these factors can give rise to a change in output voltage, V, especially if it is not caused by any string movement which can be a sticky point we will discuss at some point. So, to begin with we can basically conclude that the area of the coil will not change, and even if the coil wire should move “up and down”, it will not change area or angle in any significant way. So we have hereby killed the myth that wire movement caused by some sort of impact on the pickup, such as tapping or shouting at it will cause the voltage output to change accordingly. I will elaborate later on why I thing this myth got started!
Now, here is a different scenario, one that is highly unlikely in practice, where the coil is hit by an impact from the side of the coil actually pushing the coil inwards, which will lead to a temporary deforming of the coil, resulting in a smaller area. The time involved here is essential because it is only a change in area with time, dA/dt, that causes a change in output voltage, so this change in time must be so fast the it falls within the frequency range of the pickup, is it so slow that the it barely registers as a frequency, it will have caused no change in output voltage.
That leaves us with B, a change in B, dB/dt, we know will transfer the movement of the strings to the output of the pickup, which is what we want to hear. So the strings movement will cause a change in B, something we have already discussed in length. What other than string movement can cause a change in B leading to an output voltage of the pickup that is eventually transferred to the speaker of the amp and transmitted to our ear. Well, the above measurements with and without damping on the pickup coils have showed us that such damping that basically makes it impossible for the strings to move, if the could, has no effect. In one instant, without damping, the coil could move, but the second experiment with damping showed that the coil wires did not move in the first experiment because there was no change from one to the other in response.
As described, the pole piece was impacted and if we look at the equation and other things brought up above, there is no indication that any movement (which was prevented in this case, would cause any kind of voltage on the output. The only thing left is to ask the question: Does tapping the magnet directly or indirectly give rise to a dB/dt and that to an output of the pickup? The answer is yes, and that is where magnetostriction comes in because it is a known fact that a magnet can be de-magnetized by mechanical impact, this leads to that a mechanical impact on the magnet, or to include all, a ferromagnetic material, here in the form of a tap as described above, will be able to alter the magnetic B field in time, this will cause an impulse in the coil creating an electric voltage on the output wires. Since the pickup is a somewhat complex circuit that has a resonance frequency, the ringing will be dampened and with this frequency. All this is in perfect harmony with the experiments that have been described above. We have eliminated any other source so the strong leaning is that this is caused by magnetostriction.
The reason why the myth says that it is caused by changing in coil wire position is simple, just think of the classic but simple experiment with moving a wire in a magnetic field will cause a current to flow, that is true, but there is no analogy to what is at hand here. Even the other classic experiment where a coil rotates in an magnetic field has any bearing on this problem.
That leaves us with magnetostriction!